A genome-wide scan maps a novel autosomal dominant juvenile-onset open-angle glaucoma locus to 2p15-16.

PURPOSE
To study the clinical features and to perform genetic linkage study in two large Chinese families with autosomal dominant juvenile-onset primary open-angle glaucoma (POAG).


METHODS
Eighteen members of one Chinese family and 25 members of a second Chinese family with juvenile-onset primary open-angle glaucoma (POAG) were investigated. Thirteen members in one family and 14 members in the second family were diagnosed with juvenile-onset POAG. A genome-wide linkage scan was performed on one family using 411 short tandem repeat (STR) markers. Subsequent fine mapping was performed in the two study families using a modified fluorescent labeled M13 primer method.


RESULTS
A whole genome-wide scan in one family showed linkage to chromosome 2p15-p16 with a two-point maximum LOD score of 5.01 at theta=0 between the disease phenotype and STR marker D2S337. The second family was also mapped to the same locus with a two-point maximum LOD score of 6.30 at theta=0 for D2S378. Haplotype analysis in these two families demonstrated that they shared the same disease haplotype, suggesting they have inherited the mutation from a common founder. The maximum LOD scores were 8.93 at theta=0 for D2S378 and 9.9 at theta=0 for D2S337 when the two families were combined for analysis. The disease interval for these two families was localized to 9.2 cM or 13.3 Mb between D2S123 and D2S2397. There are 42 known genes/transcripts within the interval. Five of these genes were sequenced, and no disease-causing mutation was identified in either family.


CONCLUSIONS
This novel juvenile-onset POAG locus on chromosome 2p15-16 is overlapped by the Glaucoma 1, open angle, H (GLC1H) locus for adult-onset POAG. Eventual identification of the disease-causing gene will provide insights into the pathogenesis of POAG.

Glaucoma is a leading cause of blindness in the world [1]. The disease causes irreversible, characteristic visual field loss and optic nerve damage, usually associated with elevated intraocular pressure (IOP). Glaucoma is a heterogeneous group of optic neuropathies that can be divided into congenital, juvenile-onset, and adult-onset categories, and it can be inherited as a Mendelian autosomal-dominant, an autosomal-recessive trait, or a complex multifactorial trait. Regarding the Mendelian inherited glaucoma, congenital is only inherited as autosomal recessive. Juvenile-onset and adult-onset glaucoma are inherited as autosomal dominant. The majority of glaucoma cases (60%-70%) are associated with a normal-appearing trabecular meshwork, a visual field Correspondence to: Dr. Zhenglin Yang, Human Molecular Biology and Genetics, Sichuan Academy of Medical Sciences and Sichuan Provincial People's Hospital, 32 First Ring Road 2 West, Chengdu, Sichuan, 617002, China; Phone: 86-28-87394037; FAX: 86-28-87393596; email: zliny@yahoo.com loss, and a frequently elevated intraocular pressure (IOP). This presentation of glaucoma is termed primary open-angle glaucoma (POAG) [2]. The affected POAG patients may maintain useful sight if the disease is treated before significant damage to the optic nerve occurs. Therefore, early diagnosis is critical for vision loss prevention. Juvenile-onset openangle glaucoma is a subset of POAG that appears earlier in life, usually before 40 years old, and is inherited in an autosomal dominant manner [3]. Seven loci for adult-onset autosomal dominant POAG have been mapped, including GLC1A (Myocilin,MYOC; 1q23), GLC1B (2cen-2q13), GLC1C (3q21-24), GLC1D (8q23), GLC1G (WD repeatcontaining protein 36,WDR36) (5p22), GLC1F (7q35-q36), and GLC1H (2p15-p16) [3][4][5][6][7][8]. For juvenile-onset autosomal dominant POAG, only five loci, including GLC1A (MYOC) (1q23), GLC1J (9q22), GLC1K (20q12), 5q22.1-q32, and 15q22-q24, have been mapped, and only one gene (MYOC) has been identified [4,[9][10][11][12][13]. Mapping and identifying new loci and genes for juvenile-onset POAG will contribute to the understanding of the pathogenesis of glaucoma. Here, we report that two juvenile-onset families map to the 2p15-p16 region.  family (Family A) and 25 members were enrolled in the second family (Family B; Figure 1). All family members underwent ophthalmic examination including visual acuity testing, tonometry, gonioscopy, and visual field testing. Clinical diagnosis was based on IOP, vision field loss, angle appearance, and optical disc appearance by the ophthalmologist specializing in glaucoma from Sichuan Academy of Medical Sciences and Sichuan Provincial People's Hospital and Daping Hospital of the Third Military University of Medical Sciences. The patient was diagnosed with POAG using the following four criteria: optical damage (cup/disc ratio >0.5), visual field loss found by a Humphrey perimetry test, open-angle appearance by gonioscopy, and an IOP equal to or higher than 22 mmHg while not on medication.

METHODS
Genotyping and linkage analysis: Blood was collected by venepuncture, and genomic DNA was isolated from the samples using a PUREGENE blood kit from Gentra Systems (Biocompare Inc., San Francisco, CA). The known loci related to glaucoma, including GLC1A (1q23, MYOC), (13q14), GLC1L (15q11-q13), GLC1K (20p12), 5q22.1-q32, and 15q22-q24 [11], were examined by genotyping and linkage analysis for both families. A genome wide linkage scan was performed in Family A using ABI Linkage Mapping Set v2.5, which contained 411 short tandem repeat markers and True Allele PCR Premix (ABI, Foster City, CA) according to the manufacturer's instructions. The amplified polymerase chain reaction (PCR) products were loaded on to the ABI 3100 Genetic Analyzer (ABI, Foster City, CA). We used the MLINK of the LINKAGE program to calculate twopoint LOD scores (v.5.1; Human Genome Mapping Project Resources Center, Cambridge, UK) [14,15]. An autosomal dominant mode of inheritance with full penetrance and a disease allele frequency of 0.0001 were assumed in the calculations. For fine mapping, additional short tandem repeat (STR) markers were chosen from the Marshfield database and used for genotyping using a modified fluorescent labeled M13 primer method [16]. DNA sequence analysis: Nine genes, including MYOC, CYP1B1, WDR36, OPTN, and five genes within the interval (ASB3, GPR75, CHAC2, RPS27A, and CCDC88A) were sequenced using primers designed to amplify the complete coding regions and intron splice sites. PCR products were purified using QIAquik Gel Exaction Kit (Qiagen, Valencia, CA) and sequenced by both forward and reverse primers. The sequencing was performed using Big Dye ® Terminator v3.1 cycle sequencing kits (ABI, Foster City, CA) and was analyzed on the ABI 3100 Genetic Analyzer.    (Figure 2, Table 1). Five patients in Family A and five patients in Family B had elevated intraocular pressures and increased cup-disc ratio of the optic nerve ( Figure 2, Table 1).

Clinical features: Eighteen family members were studied in
Linkage analysis and haplotype analysis: STR markers, GLC1A (MYOC; 1q23), GLC1J (9q22), GLC1K (20q12), 5q22.1-q32, and 15q22-q24, encompassing previously known loci related to juvenile-onset POAG, were genotyped first.  an LOD score of 5.01 at D2S337. Additional STR makers were genotyped, and the disease interval was narrowed down to 9.2 cM between D2S123 and D2S2397 using refining STR markers and haplotype analysis. Subsequently, genotype and linkage analysis was performed for Family B in this region. We found that Family B also showed complete linkage to the same region with the same (9.2 cM) interval between D2S123 and D2S2397. A maximum two-point LOD score of 5.01 was obtained at θ=0 for D2S337 in Family A and 6.30 at θ=0 for D2S378 in Family B. On the basis of allele size and haplotype analysis, the two families share the same disease haplotype, suggesting they have inherited the same mutation from a common founder. The maximum LOD scores were 8.93 at θ=0 for D2S378 and 9.9 at θ=0 for D2S337 when the two families were combined for analysis. The disease interval was defined to 9.2 cM between D2S123 and D2S2397 in these two Chinese families (Figure 1 and Figure 3). Table 2 shows the two-point LOD scores of the STR markers around the linkage site.
Sequence analysis: There were 42 known or predicted genes within the interval (NCBI). No disease-causing mutation was identified in any of the five genes in this interval (ASB3, GPR75, CHAC2, RPS27A, and CCDC88A) after sequencing analysis.
Here, we describe a new juvenile-onset POAG locus on 2p15-p16 linked to two large Chinese families. Interestingly, this early-onset POAG locus partially overlapped with an adult-onset POAG locus (GLC1H) reported recently [8] ( Figure 3). This locus is next to a previously described adultonset locus on chromosome 2 between D2S441 and D2S2232 [22], suggesting the possibility of the same gene causing both early-onset and adult-onset glaucoma as MYOC does [4,17,23]. The interval of GLC1H is 8.3 Mb between D2S2352 and D2S2165, which is within our interval [8].
An investigation of monogenic glaucoma may help elucidate the pathogenic mechanisms of late-onset complex glaucoma, which involves multiple genes and environmental factors. Complex glaucoma affects many more people, and no major gene has yet been identified for complex primary glaucoma. However, recently the LOXL1 gene has been shown to be associated with exfoliation glaucoma (XFG), a secondary glaucoma to exfoliation syndrome (XFS) [24][25][26]. Because juvenile-onset glaucoma, which is associated with single-gene mutations, shares clinical and histopathologic features with adult-onset glaucoma, monogenic glaucoma genes like MYOC and WDR36 may be associated with complex glaucoma. Identification of the disease-causing gene(s) in this 2p15-p16 locus linked to both early-onset and late-onset glaucoma has the possibility of finding a common pathway to these diseases and defining the underlying pathophysiology. Additionally, gene discovery may lead to an early DNA-based diagnosis test, which may contribute to therapeutic interventions at early stages of the disease and preserve vision. The disease interval for family A is between D2S123 and D2S2397 with the maximum LOD score of 5.01 for D2S337 at θ=0, and the disease interval for family B is between D2S123 and D2S380 with the maximum LOD score of 4.89 for D2S337 at θ=0.